An alkaloid is a nitrogenous organic molecule that has a pharmacological effect on humans and other animals. The name derives from the word alkaline; originally, the term was used to describe any nitrogen-containing base (an amine in modern terms). Alkaloids are found as secondary metabolites in plants (e.g., in potatoes and tomatoes), animals (e.g., in shellfish) and fungi, and can be extracted from their sources by treatment with acids (usually hydrochloric acid or sulfuric acid, though organic acids such as maleic acid and citric acid are sometimes used).

Even though many alkaloids are poisonous (such as strychnine or coniine), some are used in medicine as analgesics (pain relievers) or anaesthetics, particularly morphine and codeine. Most alkaloids have a very bitter taste.

Alkaloid classifications

Alkaloids are usually classified by their common molecular precursors, based on the biological pathway used to construct the molecule. When not much was known about the biosynthesis of alkaloids, they were grouped under the names of known compounds, even some non-nitrogenous ones (since those molecules' structures appear in the finished product; the opium alkaloids are sometimes called "phenanthrenes", for example), or by the plants or animals they were isolated from. When more is learned about a certain alkaloid, the grouping is changed to reflect the new knowledge, usually taking the name of a biologically-important amine that stands out in the synthesis process.

• Pyridine group: piperine, coniine, trigonelline , arecaidine ; guvacine ; pilocarpine; cytisine ; nicotine; sparteine .

• Tropane group: Atropine, cocaine, hygrine , ecgonine , pelletierine .

• Quinoline group: quinine, strychnine, brucine, veratrine , cevadine

• Isoquinoline group: The opium alkaloids (morphine, codeine, thebaine, papaverine, narcotine , narceine , hydrastine , berberine )

• Phenethylamine group: MDMA, methamphetamine, mescaline, ephedrine

• Indole group:
  • Tryptamines: DMT, NMT , psilocybin, serotonin
  • Ergolines: the ergot alkaloids (ergine, ergotamine, lysergic acid, etc.), LSD
  • Beta-carbolines: harmine, yohimbine, reserpine, emetine

• Purine group:
  • Xanthines: caffeine, theobromine, theophylline

• Terpenoid group:
  • Aconite alkaloids: aconitine
  • Steroids: solanine

• Betaines (quaternary ammonium compounds): muscarine, choline, neurine